In the United States, the broadcast FM radio band occupies a band of frequencies extending from approximately 88 MHz to approximately 108 MHz. This band of frequencies lies between the frequencies allocated for broadcast television channel 6 and television cable channel 98. In addition, the National Weather Service broadcasts FM radio signals in a band of frequencies near 162.5 Mhz. Single-tuner multiband radio receivers having the capability to receive AM, broadcast FM, and National Weather Service radio signals, and television sound signals, are known from, for example, the Windsor Radio, model number 2239, FCC ID number BGK91F2239, manufactured in Hong Kong.
Television receivers having the capability to receive broadcast FM signals are known from the prior art. The Dumont model RA-119A television receiver, manufactured by Allen B. Dumont Laboratories, Passaic, N.J., is an example of a class of television receivers having a single tuner for receiving both television signals and broadcast FM radio signals. This single-tuner class of television receivers was manufactured during the years 1949 to 1952 inclusive, and utilized a continuous tuning arrangement for tuning signals having frequencies between 44 MHz and 217 MHz. This class of television receivers employed a so-called split sound IF system, that is, a separate sound IF channel tuned to 41.25 Mhz. The sound signals for both television and FM radio were demodulated directly from signals at the television sound IF frequency of 41.25 MHz.
Modern television receivers have abandoned the split-sound IF system in favor of the intercarrier sound system, which is less complex, less costly, and more reliable. The intercarrier-sound IF system takes advantage of the fact that the picture and sound carriers are held to close tolerances at the transmitter, ensuring that they are always separated by a constant 4.5 MHz. In an intercarrier-sound IF system, the sound IF signals are amplified along with the picture IF signals in a single IF amplifier. After amplification, the sound signals are converted to a 4.5 MHz intercarrier sound IF frequency by "beating" (i.e., heterodyning) the sound IF signals at 41.25 MHz against a signal at the picture carrier frequency of 45.75 MHz. The television sound signals are then demodulated from the resulting 4.5 MHz intercarrier signal.
Modern intercarrier-sound type television receivers cannot recover and reproduce broadcast FM radio sound signals, because FM radio signals lack a signal at the picture carrier frequency required by the television receiver circuitry for use in recovering the sound signal. Therefore, as a result of adopting the intercarrier sound system, television manufacturers wanting to provide the capability of receiving broadcast FM radio signals, added a separate FM radio having its own tuner.
Copending U.S. Pat. Ser. No. 561,588, entitled STEREO FM RADIO IN A TELEVISION RECEIVER (Wignot et al.) discloses a double conversion FM radio in which the first frequency conversion stage is the television tuner. Copending U.S. patent application Ser. No. 561,586, entitled NATIONAL WEATHER RADIO RECEPTION BY SYNTHESIZING ONLY THE CENTER FREQUENCY (Wignot et al.) discloses a system in which a double conversion FM radio using a television tuner as the first conversion stage can receive both FM radio broadcast signals and signals transmitted by the National Weather Service, both of which are processed by the same circuitry, including the same discriminator circuit.
Unfortunately, using the same discriminator circuit for demodulation of both FM broadcast signals and National Weather Service signals causes a problem in that the amplitudes of the recovered signals at the output of the discriminator circuit are vastly different. Recall that the amplitude of the demodulated audio signal is a function of the deviation of the FM carrier frequency from its nominal center frequency. Specifically, the vastly differing amplitude problem occurs because the peak deviation of broadcast FM stereo radio signals is approximately 68 kHz, while the peak deviation of the National Weather Service FM radio signals is only 5 kHz. The difference in the magnitudes of these two signals is equal to 20 log (68/5)=22.67 dB. This situation would force the listener to greatly increase the volume when the receiver is tuned to a National Weather Service station, and to greatly reduce the volume when tuning back to a broadcast FM radio station.